1. Field of the Invention
This invention relates to a radiation image detecting system, and more particularly to suppression of deterioration in image quality due to backscattering of radiations and/or protection of a latent image charge detecting circuit against the radiations in a radiation image detecting system.
2. Description of the Related Art
There has been known a radiation image information read-out system using radiographic film or a stimulable phosphor sheet in medical radiography.
Recently there has been proposed a radiation image detecting system employing a solid radiation detector including a semiconductor as a major part which detects radiations and converts the intensity of radiations to an electric signal. Though various types of solid radiation detectors have been proposed, the following solid radiation detectors are representative.
That is, a solid radiation detector comprising a two-dimensional image reader formed by two-dimensionally forming a plurality of photoelectric conversion elements (each forming a picture element) on an insulating substrate and a phosphor layer (scintillator) which is formed on the two-dimensional image reader and generates visible light bearing thereon image information when exposed to radiations bearing thereon image information. The solid radiation detector of this type will be referred to as xe2x80x9ca photo-conversion type solid radiation detectorxe2x80x9d, hereinbelow.
A solid radiation detector comprising a two-dimensional image reader formed by two-dimensionally forming a plurality of charge collection electrodes (each forming a picture element) on an insulating substrate and a radiation-conductive material layer which is formed on the two-dimensional image reader and generates electric charges bearing thereon image information when exposed to radiations bearing thereon image information. The solid radiation detector of this type will be referred to as xe2x80x9ca direct conversion type solid radiation detectorxe2x80x9d, hereinbelow.
The photo-conversion type solid radiation detectors are disclosed, for instance, in Japanese Unexamined Patent Publication Nos. 59(1984)-211263 and 2(1990)-164067, PCT International Publication No. WO92/06501, xe2x80x9cSignal, noise, and read out considerations in the development of amorphous silicon photodiode arrays for radiography and diagnostic x-ray imagingxe2x80x9d, L. E. Antonuk et. al., University of Michigan, R.A. street Xerox, PARC, SPIE vol. 1443, xe2x80x9cMedical Imaging Vxe2x80x9d, Image Physics (1991), pp. 108-119, and the like.
In the photo-conversion type solid radiation detector, the photoelectric conversion elements have also function of storing detected electric charges, and the electric charges obtained by photoelectric conversion are stored in the photoelectric conversion elements as latent image charges.
As the direct conversion type solid radiation detector, the following have been proposed.
1) A solid radiation detector which is about ten times as large as normal solid radiation detectors in thickness as measured in the direction in which radiations are transmitted through the solid radiation detector. See xe2x80x9cMATERIAL PARAMETERS IN THICK HYDROGENATED AMORPHOUS SILICON RADIATION DETECTORSxe2x80x9d, Lawrence Berkeley Laboratory. University of California, Berkeley, Calif. 94720 Xerox Parc. Palo Alto. Calif. 94304.
2) Those comprising a plurality of solid radiation detectors laminated in the direction in which radiations are transmitted with metal plates intervening therebetween. See xe2x80x9cMetal/Amorphous Silicon Multilayer Radiation Detectorsxe2x80x9d, IEE TRANSACTIONS ON-NUCLEAR SCIENCE. VOL. 36. NO.2 APRIL 1989.
3) Solid radiation detectors using CdTe and the like disclosed in Japanese Unexamined Patent Publication No. 1(1989)-216290.
In the direct conversion type solid radiation detector, a capacitor is connected to each of the charge collection electrodes and the electric charges collected by the charge collection electrodes are stored in the capacitors as latent image charges.
This applicant has proposed an improvement on the direct conversion type solid radiation detector as disclosed in Japanese Patent Application 9(1997)-222114. The solid radiation detector will be referred to as xe2x80x9can improved direct conversion type solid radiation detectorxe2x80x9d, hereinbelow.
The improved direct conversion type solid radiation detector comprises a first conductive layer which is transparent to recording radiations, a recording photoconductive layer which exhibits photoconductivity upon exposure to the recording radiations passing through the first conductive layer, a charge transfer layer which acts substantially as an insulator to electric charges of the same polarity as that in which the first conductive layer is charged and as a conductor to electric charges reverse to that in which the first conductive layer is charged, a read-out photoconductive layer which exhibits photoconductivity upon exposure to read-out electromagnetic waves, and a second conductive layer which is transparent to the read-out electromagnetic waves. These layers are superposed one on another in this order and latent image charges are collected on the interface between the recording photoconductive layer and the charge transfer layer.
As a system for reading out the latent image charges in the improved direct conversion type solid radiation detector, there may be employed a read-out system where the read-out electrode (the second conductive layer) is made like a flat plate and the latent image charges are read out by scanning the read-out electrode with a read-out light spot such as a laser beam, or a read-out system where the read-out electrode is made like a stripe electrode (comb tooth electrode) and the latent image charges are read out by scanning the stripe electrode with a line light beam, extending in a direction perpendicular to the longitudinal direction of the stripe electrode, in the longitudinal direction of the stripe electrode. Irrespective of whichever read-out system is employed, the solid radiation detector forms a two-dimensional image read-out system in which a plurality of detecting elements (each forming a picture element) formed by the charge transfer layer, the read-out photoconductive layer and the second conductive layer are two-dimensionally arranged.
In any one of the solid radiation detectors described above, a detecting circuit which converts the latent image charges to an image signal is connected to the solid radiation detector. The detecting circuit converts the latent image charges stored on the photoelectric conversion elements, the charge collection electrodes or the detecting elements to an image signal, and the image signal is output after subjected to a predetermined image processing, and is reproduced as a visible image by a reproducing system such as a CRT.
In radiography employing such a solid radiation detector, radiations which have passed through an object and bear thereon transmission radiation image information of the object are caused to impinge upon the solid radiation detector, and the radiations are converted to latent image charges bearing thereon the transmission radiation image information of the object in the solid radiation detector, and the latent image charges are stored in the solid radiation detector. Thereafter the latent image charges in the solid radiation detector is converted to an image signal by a detecting circuit and the image signal is output.
Not all the radiations impinging upon the solid radiation detector are converted to the latent image charges but a part of the radiations passes through the solid radiation detector and impinges upon substance behind the solid radiation detector. The radiations reflected by the substance behind the solid radiation detector reenter the solid radiation detector from rearward as backscattering radiations bearing thereon image information on the substance and are converted to latent image charges bearing image information on the substance.
As a result, the solid radiation detector stores both the latent image charges representing the image information on the substance behind the solid radiation detector and the latent image charges representing the transmission radiation image information on the object. Accordingly, the image signal finally output from the detecting circuit includes components representing the image information on the substance behind the solid radiation detector, which deteriorates quality of the image.
Further the detecting circuit is generally formed by an integrated circuit integrally with the solid radiation detector. When the detecting circuit is exposed to the radiations during taking a radiation image, components of the detecting circuit, for instance, amplifiers and/or memories, can malfunction and the service life of the components can be shortened.
In view of the foregoing observations and description, the primary object of the present invention is to provide a radiation image detecting system in which deterioration in image quality due to backscattering of radiations can be suppressed.
Another object of the present invention is to provide a radiation image detecting system in which components of the detecting circuit cannot malfunction and the service life of the components cannot be shortened due to exposure to the radiations during taking a radiation image.
The radiation image detecting system in accordance with the present invention is characterized in that a radiation absorbing member is provided on the side of the solid radiation detector remote from the recording radiation source so that the recording radiation is absorbed by the radiation absorbing member and no backscattering radiation is generated.
That is, in accordance with a first aspect of the present invention, there is provided a radiation image detecting system comprising
a solid radiation detector (of the improved direct conversion type) comprising a first conductive layer which is transparent to recording radiations, a recording photoconductive layer which exhibits photoconductivity upon exposure to the recording radiations passing through the first conductive layer, a charge transfer layer which acts substantially as an insulator to electric charges of the same polarity as that in which the first conductive layer is charged and as a conductor to electric charges reverse to that in which the first conductive layer is charged, a read-out photoconductive layer which exhibits photoconductivity upon exposure to read-out electromagnetic waves, and a second conductive layer which is transparent to the read-out electromagnetic waves, which layers are superposed one on another in this order, and
a radiation absorbing member which is formed of material absorbent of the recording radiations and is provided on the second conductive layer side of the solid radiation detector.
It is preferred that the radiation image detecting system be provided with a detecting circuit which is provided on the side of the radiation absorbing member remote from the solid radiation detector and detects latent image charges collected on the interface between the recording photoconductive layer and the charge transfer layer.
In accordance with a second aspect of the present invention, there is provided a radiation image detecting system comprising
a solid radiation detector (of the direct conversion type) comprising a two-dimensional image reader formed by two-dimensionally forming a plurality of charge collection electrodes, each forming a picture element, on an insulating substrate and a radiation-conductive material layer which is formed on the two-dimensional image reader and generates electric charges bearing thereon image information when exposed to radiations bearing thereon image information, and
a radiation absorbing member which is formed of material absorbent of the radiations and is provided on the insulating substrate side of the solid radiation detector.
It is preferred that the radiation image detecting system be provided with a detecting circuit which is provided on the side of the radiation absorbing member remote from the solid radiation detector and detects charges collected by the charge collection electrodes.
In accordance with a third aspect of the present invention, there is provided a radiation image detecting system comprising
a solid radiation detector (of the photo-conversion type) comprising a two-dimensional image reader formed by two-dimensionally forming a plurality of photoelectric conversion elements, each forming a picture element, on an insulating substrate and a phosphor layer which is formed on the two-dimensional image reader and generates visible light bearing thereon image information when exposed to radiations bearing thereon image information, and
a radiation absorbing member which is formed of material absorbent of the radiations and is provided on the insulating substrate side of the solid radiation detector.
It is preferred that the radiation image detecting system be provided with a detecting circuit which is provided on the side of the radiation absorbing member remote from the solid radiation detector and detects electric charges obtained by photoelectric conversion by the photoelectric conversion elements.
Preferably the radiation absorbing member is 0.1 mm to 10 mm in thickness and more preferably is 0.2 mm to 1.0 mm in thickness.
In the radiation image detecting system of the present invention, the radiation absorbing member reduces the backscattering radiations and accordingly deterioration in image quality due to the backscattering radiations can be suppressed, whereby a high quality image relatively free from influence of the backscattering radiations can be obtained.
Further when the detecting circuit is disposed behind the radiation absorbing member, the detecting circuit is protected from radiations and malfunction and/or reduction in service life of the detecting circuit can be prevented.